Getting weird volume fog colouring with physical cam values

I have a simple scene with a single rect light casting light into an opening of cubes. I have a lightshaft coming through the opening when I use an exponential height fog and enable volume fog.

when I just use standard auto exposure, and I colour the light using a 2000 color temp, this looks fine as as expected.(see below)

When I apply physical camera exposure values in the ppv

image

which then means I need to adjust the light intensity, to maintain exposure, I start getting a multiple color bleeding (this green tint). This is not the desired result. NOTE this doesn’t happen with a directional light, just spot, and rect lights. It could be related to inverse square falloff

Does anyone know what is causing this? Is this a bug? Suggestions?